use:

 

package{

   

    import flash.display.Sprite;

    import flash.events.MouseEvent;

    import flash.net.navigateToURL;

    import flash.net.URLRequest;

    import flash.display.Loader;

    public class Main extends Sprite{

       

        private var _loader:Loader = new Loader();

       

        public function Main():void{

            mc_star.addEventListener(MouseEvent.CLICK,buttonClick);

            mc_gear.addEventListener(MouseEvent.CLICK,buttonClick);

            mc_apple.addEventListener(MouseEvent.CLICK,buttonClick);

            mc_photo.addEventListener(MouseEvent.CLICK,buttonClick);

            mc_buy.addEventListener(MouseEvent.CLICK,buttonClick);

            mc_contact.addEventListener(MouseEvent.CLICK,buttonClick);

            //_loader=new Loader();

            addChild(_loader);

        }

       

        private function buttonClick(e:MouseEvent):void{

            var url:URLRequest=new URLRequest();

            switch(e.target.name){

                case "mc_star": url.url="bg2.swf"; break;

                case "mc_gear": url.url="bg3.swf"; break;

                case "mc_apple": url.url="bg4.swf"; break;

                case "mc_photo": url.url="bg5.swf"; break;

                case "mc_buy": url.url="bg6.swf"; break;

                case "mc_contact": url.url="bg7.swf"; break;

            }

            _loader.load(url);

        }

    }   

}

click file/publish settings/swf and tick "permit debugging".  retest.

the line number with the null object will be in the error message.  that will probably be one of your mc_ objects.

when you find which, return to your fla's stage and click the object you think should have that instance name and fix the problem.

which is line 23?

mc_star doesn't exist.

return to your fla's stage and click the object you think is mc_star and fix the problem.

you're welcome.

p.s.  please mark helpful/correct responses.